This invention relates generally to a method of forming a dielectric of the type used in integrated circuits, and more particularly to a plasma enhanced chemical vapor deposition method of forming a low-dielectric-constant insulating material.
The designers and makers of large scale integrated circuits continue to make ever-smaller devices, which allow for greater speed and increased device packing densities. Sizes of individual features (e.g. the transistor gate length) on ultra-large-scale-integration (ULSI) circuits is shrinking to less than 200 nanometers (nm). The resultant increase in packing densities on a semiconductor chip, and the associated increase in functionality, has greatly increased the number and density of interconnects on each chip.
Smaller on-chip devices, packed closer together, with increased functionality and complexity, require interconnects (lines, vias, etc.) which are smaller, more complex (e.g. more wiring levels), and more closely spaced. The smaller sizes of the interconnects, which increases resistance, and closer interconnect spacing, which increases capacitance, produces RC (resistance-capacitance) coupling problems including propagation delays, and cross talk noise between interlevel and intralevel conductors. As interconnect lines, both interlevel and intralevel, become smaller and more closely spaced, RC delays become an increasing part of total signal delays, offsetting much of the speed advantage derived from smaller device size. RC delays thus limit improvement in device performance. Small conductor size increases the resistivity (R) of metal lines and smaller interline and interlevel spacing increases the capacitance (C) between lines. Use and development of lower-resisitivity metals such as copper continue to reduce the resisitivity of interconnect lines. However, it is also important to reduce the capacitance.
Since capacitance (C) is directly proportional to the dielectric constant (k) of the interconnect dielectric, capacitance can be reduced by employing lower dielectric constant (low-k) materials. Lowering the capacitance (C) helps to reduce the problems associated with RC coupling in ULSI circuits. What the industry is looking for is a suitable replacement for silicon dioxide (SiO2), which has long been used as a dielectric in integrated circuits. Silicon dioxide has excellent thermal stability and relatively good dielectric properties, having a dielectric constant of around 4.0. But there is now a need for an interconnect dielectric material which is suitable for use in integrated circuit interconnects and which has a lower dielectric constant than SiO2.
After a long search for possible low-k materials to be used as interconnect dielectrics in ULSI circuits, the candidates have been narrowed down to a few, depending upon the application. One of the promising materials is carbon-doped, or carbonaceous, silicon oxide (SiOC).
Carbonaceous silicon oxide has been successfully deposited by chemical vapor deposition (CVD) using methylsilane based precursors, such as trimethylsilane or tetramethylsilane. Methylsilane based SiOC materials have dielectric constants of between approximately 2.5 and 3.1. However, methlysilane based precursors are organosilicon compounds that are extremely volatile and flammable. They are also expensive. These various drawbacks make the use of methylsilane based precursors less than ideal for the processing of integrated circuit devices.
It would be advantageous to have a method of forming SiOC without the use of methylsilane based precursors.
It would be advantageous to have a method of forming SiOC using readily available precursor materials and equipment.
Accordingly, a method of forming an interconnect dielectric material for a semiconductor device is provided. The method comprises the steps of:
a) positioning a substrate within a plasma enhanced chemical vapor deposition (PECVD) chamber; and
b) introducing a source of silicon, a source of oxygen, and a source of carbon into the deposition chamber under sufficient applied energy to form plasmas of silicon containing free radicals, oxygen containing free radicals, and carbon containing free radicals, whereby silicon, oxygen, and carbon are available to form a carbonaceous silicon oxide (SiOC) film. In a preferred embodiment of the present invention, the source of silicon is silane, the source of oxygen is nitrous oxide or oxygen, and the source of carbon is methane, acetylene or both. These precursors are readily available and produce materials with dielectric constants of less than four (4.0). In a preferred embodiment using both methane and acetylene, the method produces materials with dielectric constants of less than three (3.0).
Materials with lower density and lower dielectric constants are produced by introducing more network terminating species and reducing the number of bridging species. The reduction in dielectric constant can be achieved by doping methyl (xe2x80x94CH3) groups into an SiO2 network to form SiOC, where some Sixe2x80x94O bonds are replaced by Sixe2x80x94CH3 bonds.
Carbonaceous silicon oxide (SiOC) refers herein to a material comprising silicon oxygen and carbon, additional materials may be present and may even be desirable, especially hydrogen, which forms the desirable methyl group. For instance in a preferred embodiment of the present method, an interconnect dielectric material is produced with the carbon component being provided by a methyl group (xe2x80x94CH3), to form a structure akin to H3Cxe2x80x94SiO3.
In a preferred embodiment of the present invention, methane and acetylene are both introduced into the plasma chamber. The plasma energy within the chamber converts methane into methyl (xe2x80x94CH3), which is a network terminating free radical, or species, and carbene (xe2x80x94CH2xe2x80x94), which is a bridging free radical, or species. Species herein refers to the free radical while it remains free within the plasma and also once it has bonded when forming the material. Since structures with network terminating species (xe2x80x94CH3), such as: 
produce lower density and lower dielectric constant materials, they are preferred. Structures incorporating bridging species, which tend to form undesirable amorphous silicon carbide type linkages, such as: 
will preferably be reduced or eliminated.
In another preferred embodiment of the method of the present invention, gas precursors are introduced into the plasma chamber. These gas precursors include a silicon precursor, preferably silane, an oxygen precursor, preferably nitrous oxide, a network terminating precursor, preferably methane, and a modifier precursor, preferably acetylene. While both methane and acetylene contain carbon and act as a possible source of carbon, the combination is preferred. The methane provides a network terminating free radical, methyl (xe2x80x94CH3), along with a less desirable bridging free radical, carbene (xe2x80x94CH2 xe2x80x94). The presence of a modifier precursor, in this case acetylene, either enhances production of network terminating species or reduces the undesirable species, which allows a larger portion of methyl to replace Sixe2x80x94O bonds producing lower-k materials.
Following the processing within the plasma chamber the substrate is annealed to condition the film by reducing xe2x80x94OH, as well.